Motor vehicles have incorporated vehicle air conditioning systems involving a compressor driven by the vehicle engine. In large multi-passenger vehicles such as buses, attempts have been made to employ helical screw rotary compressor driven air conditioning systems with the helical screw compressor being mechanically driven by the bus diesel engine or the like through a conventional belt drive. The helical screw rotary compressor incorporates a longitudinally shiftable slide valve for loading and unloading the compressor, wherein the slide valve, which forms a part of the envelope for the compression process, shifts longitudinally to permit more or less of the compressor working fluid in gaseous form to return uncompressed to the suction side of the machine, depending upon system requirements. Further, conventionally, the slide valve itself is mechanically coupled to a linear fluid motor or slide valve actuator such as a piston slidable within a cylinder. Compressed working fluid may be applied to one face of the piston remote from the slide valve, while the opposite face is open to the compressor discharge pressure. As such, the compressor discharge pressure tends to shift the slide valve towards an extreme unload position, while the application of high side discharge working fluid to the opposite side of the piston causes the piston to force the slide valve towards full load position.
Further, in order to control desired loading and unloading, the fluid line leading to the cylinder chamber on the side of the piston opposite that bearing compressor discharge gas, may be selectively opened to the compressor discharge gas or to suction gas, that is, the high and low side working fluid, by suitable load and unload solenoids within respective lines leading from that cylinder to the discharge and suction gas sides of the compressor itself.
This control scheme is not only applied to vehicle air conditioning systems employing engine driven helical screw rotary compressors, but in gas systems generally, which employ a helical screw rotary compressor slide valve for controlled loading and unloading of the compressor. However, in order to insure that the vehicle engine is not overloaded during periods of necessary acceleration, there exists the requirement of some form of compressor slide valve response to varying bus engine speed. As the bus engine accelerates, the compressor must unload essentially immediately, while as the bus engine decelerates, the compressor may be loaded essentially immediately without adverse effect to the vehicle operation.
It is, therefore, an object of the present invention to provide an improved vehicle engine driven air conditioning system which permits rapid compressor unloading during bus engine acceleration and instantaneous momentary compressor loading during bus engine deceleration.
It is a further object of the present invention to provide such improved air conditioning system involving a helical screw rotary compressor bearing an unloading slide valve which is responsive to changing evaporator temperature or changing condenser temperature to provide the high rate of response desired during engine RPM change.